High angular resolution x-ray collimator

ABSTRACT

A method of producing a high angular resolution collimator implemented in an inspection system for detecting the presence of selected crystalline materials, such as explosives or drugs. The system includes an x-ray source and an array of energy dispersive detectors to sense radiation scattered by the objects being inspected. The collimator includes a bundle of optical fibers bonded together to form a stack of plates having a plurality of microcapillaries therein to pass an x-ray beam therethrough. The method includes the steps of stacking the plates, aligning the plates in registration, and etching an inner core without disrupting registration.

BACKGROUND OF THE INVENTION

This invention relates generally to the field of radiographic detectionsystems, and more particularly to coherent x-ray scattering systemsusing a high resolution x-ray collimator to detect the presence ofexplosive materials and illicit narcotic substances.

Numerous screening systems have been developed for inspecting cargo suchas bags, suitcases, and briefcases at airports and at other secureinstallations. Of particular concern in the development of such systemshas been the detection of concealed weapons, explosives or drugs whosetransport is restricted. Unfortunately, many of these illicit materialsdo not conform to an easily identifiable shape and are not visuallydetectable in the currently used systems. In particular, many types ofexplosive materials can be molded into any shape and are not detectableby standard x-ray equipment. Typically, a conveyor transports the itemsto be inspected into and out of a chamber positioned between an x-raysource. The x-ray source, which comprises a shaped x-ray beam,irradiates the object of interest. Then an array of detectors is used tomeasure the transmitted intensity. A monitor displays an image of thesescanned items. The outline is visually inspected to determine thepresence of the objects of concern. This type of conventional x-rayimaging system provides good spatial resolution but is not capable ofdetermining the intrinsic chemical composition of the items in the cargopassing through.

SUMMARY OF THE INVENTION

In order to detect contraband such as explosive materials and illicitdrugs, systems employing energy dispersive detectors and radiationcollimators are needed. U.S. Pat. No. 5,007,072, issued to Ion TrackInstruments on Apr. 9, 1991, discloses such an x-ray inspection systemutilizing energy dispersive detectors and radiation collimators. Apolychromatic x-ray source is used to irradiate a piece of luggage. Theintensity of the diffracted rays are measured simultaneously at a fixedangle of about 2° relative to the primary beam being emitted from thex-ray source. In order for the energy dispersive detector to measure theintensity of the diffracted rays simultaneously at a fixed angle, acollimator must be employed to provide a sufficiently high resolution.

Typical radiation collimators have been used to perform gamma rayspectroscopy in areas such as nuclear medicine. In this field, largegamma ray cameras are used to obtain images of patient's internalorgans. These collimators are constructed from lead and generally have a"honeycomb" appearance. The x-rays pass through these open "honeycomb"areas at a solid angle and are measured by detectors. The x-rays whichimpinge at angles outside the angular resolution of the collimator areabsorbed in the lead walls. The typical resolution of these collimatorsis only a few degrees of arc. The effectiveness of the x-ray detectorsystem disclosed in U.S. Pat. No. 5,007,072 requires a substantiallyimproved collimator with a resolution about 100 times greater than theconventional "honeycomb" collimator. Thus, there is a need for acollimator with greater resolution.

In accordance with a preferred embodiment of the present invention,there is provided a collimator made from a bundle of optical fibersbonded together to form a solid core. The solid core has an inner coreand an outer core. The inner core has a plurality of pores or channelsextending through it that have been formed by removing the center ofeach optical fiber. Each channel provides an optical path for theradiation that is between 10 and 20 mm in length where the channeldiameter on the order of about 10 microns. Thus the collimator has anaspect ratio of about 1500. In a preferred embodiment the solid core issliced along its length, defining a stack of plates with the pores ofeach plate aligned in registration ensuring passage of an x-ray beamtherethrough. A housing securing the stack of plates preserves theregistration of the pores in each plate relative to the other plates toprovide an optical path through each channel of the entire stack.

Pursuant to another preferred embodiment of the present invention, thereis provided an x-ray diffraction inspection system for detecting thepresence of selected crystalline materials. A light source irradiates anobject with an x-ray beam. A collimator excludes unwanted x-raysscattered from the object. The collimator comprises a plurality ofplates stacked over each other as described above where each plate isformed from a bundle of bonded glass fibers. The stack of plates have aninner core and an outer core. The inner core has a plurality of holesaligned in registration to ensure passage of x-rays therethrough. Ahousing encloses the stack of plates to further ensure registration. Adetector measures the intensity of scattered light passing through thecollimator.

The present invention further includes a preferred method of fabricationfor producing a collimator comprising several steps. First, bundle ofoptical fibers are fused together. Next, the bundle of optical fibers ispartially or completely cut along its length to make plates where thebundle of cut fibers are aligned in registration. The cores of theoptical fibers extending through an inner core of the bundle are etchedwithout disrupting registration to provide a large number of channelsextending through each plate. Adjacent plates are positionedsufficiently close to each other to prevent any substantial drop inintensity of the signal. The distance between adjacent plates isgenerally less than 1.0 mm, and preferably less than 0.5 mm.Alternatively, the wafers can be completely cut, etched and then mountedinto a housing in which the channels are aligned. One or more groovescan be cut longitudinally along the side of the outer core before theplates are separately cut. The groove can be used to align the stackedplates after processing.

The above and other features of the invention including various noveldetails of construction and combination of parts will now be moreparticularly described with reference to the accompanying drawings andpointed out in the claims. It will be understood that the particularcollimator embodying the invention is shown by way of illustration only,not as a limitation of the invention. The principals and features ofthis invention may be employed in varied and numerous embodimentswithout departing form the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematical front view depicting an x-ray diffractioninspection system.

FIG. 2 is a side view of the system shown in FIG. 1.

FIG. 3 is a top view of a microchannel plate.

FIG. 4 is a side view of a microchannel plate.

FIGS. 5a-5d are side views showing the process of producing the highangular resolution collimator.

FIG. 6 is a side view of the high angular resolution collimator.

FIG. 7a is another preferred embodiment employing a flat groove foralignment.

FIG. 7b is another embodiment with a curved groove.

FIG. 7c illustrates a stacked configuration of plates with the groovesaligning the plates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A schematic illustration of the x-ray diffraction inspection systemincorporating the features of the present invention is shown in FIGS. 1and 2. This system has sufficient speed of response to detect explosivesand illicit drugs in bags being conveyed through a detection zone in amatter of seconds. X-rays from source 10 are arranged in a beam 12having a fan pattern to irradiate a bag 14 which is conveyed alongconveyor 16 through the beam 12. The beam 12 comprises an x-raycontinuum whose range of photon energies is sufficient to penetratelarge checked bags. The beam 12 is produced by collimation of the singlex-ray source 10 of constant potential with slit collimator 20. Thepolychromatic beam of x-ray photons impinges on the material under testand diffracted intensities are measured at a fixed angle, 2θ, withrespect to the incident beam using an array of energy dispersivedetectors 32.

The detection system of the present invention is comprised of energydispersive x-ray detectors 32 arranged to measure the coherent elasticscattering of x-ray photons from the lattices of crystalline materials.Such crystalline material comprise crystalline explosives and narcoticor hallucinogenic drugs. Nearly all of the explosives of interestcomprise crystalline powders. For example, the plastic explosives aremanufactured from crystalline powders of cyclotrimethyline-trinitramine(RDX), cyclotetramethyline tetranitramine (HMX) and pentaerithritoltentranitrate (PETN), and are compounded into a putty with minor amountsof organic binders. Each of the explosives when detected provide aunique diffraction pattern when irradiated with x-rays. Each of theseunique diffraction patterns are rapidly recognizable. The only notableexceptions are the nitro-glycerine-based dynamites. Fortunately theseexplosives are easy to detect by their vapor emissions. A vaporemissions detection system can be integrated with an x-ray diffractionsystem to form a single detection system. A discussion of howcrystalline material in the form of either an explosive or narcotic,scatter when illuminated with an x-ray source is provided in U.S. Pat.No. 5,007,072, which is hereby incorporated by reference.

The detection system 30 measures the intensity of scattered light inintervals of wavelengths over a wide range of photon energies but at afixed angle 2θ of scatter. This provides a unique fingerprint for eachtype of explosive or illicit drug. A detailed description of how thedetection system works and how the intensity of scattered light ismeasured is provided in U.S. Pat. No. 5,007,072, which is incorporatedherein by reference.

An array of individual energy dispersive detectors 32 is arranged acrossthe full width of the conveyor system and is irradiated by the x-ray fanbeam 12. This permits scanning of the whole volume of the bag 14. Thesource emits polychromatic x-rays ranging between 0-140 keV. The photonsscattered through a fixed angle of 2θ are detected and all other scatterangles are precluded by a narrow aperture collimator 34. Thus, thespectrum of x-rays emerge from the sample 14. Only those scattered at ornear an angle of 20θ are seen by the detector. In order to detectpolychromatic x-rays in this range, the array of energy dispersivedetectors 32 are made from high purity Germanium (HPGe). An alternativewould be to make the detectors from Cadium Telluride (CdTe).

It is believed that the foregoing description is sufficient for purposesof illustrating the general operation of the x-ray diffractioninspection system incorporating the features of the present inventiontherein.

Referring now to the specific subject matter of the present invention,the design and process of constructing a high angular resolutioncollimator will be described hereinafter with reference to FIGS. 3-6.

In order to design a collimator within the above-mentioned diffractionsystem there are several factors taken into consideration. For example,in detecting illicit narcotic substances there is typically a largefield of background scatter. Thus, the collimator must be designed toexclude as much unwanted scatter as possible so that the detector viewsthe diffracted energies of interest. Also, because of the nature oftypical cargo or parcels to be inspected and the interplanar spacings ofvarious narcotic substances, the collimator must be constructed frommaterials which have good stopping power to exclude scattered rays atthe higher energies. Another consideration is that the collimator shouldprovide angular resolution which far exceeds the resolution of standardcollimators.

To design a collimator in accordance with the above considerations,Bragg's Law is used. The most familiar form of Bragg's Law is definedas:

    nλ=2d sinθ                                    (1)

where λ is the wavelength of the incident beam (related to the energy byhc/λ), d is the interplanar spacing between the lattice planes of thecrystal (the polycrystal) under study, and θ is the angle in which thediffracted beam emerges relative to the incident beam. An application ofBragg's Law in detecting a narcotic substance such as cocaine is setforth below. The same calculations could be performed for detectingexplosive substances.

Typically, HPGe detectors exhibit about 1 keV of resolution at a beamenergy of about 100 keV. The d-spacing for cocaine is 3.315. The angleof detection, θ, is set shallow, 2(degrees), so that the diffracted raysemerge with enough energy to penetrate the cargo. These parameters causediffracted rays from the cocaine substance to emerge at 53.5 keV. Tomake full use of the HPGe detector, the angular resolution of thecollimator is determined by plugging in 54.5 keV into the Bragg equationand working backwards to determine the angle of diffraction. The angleof diffraction is determined to be 1.96° or an angular deviation of0.04°. This angular deviation which is represented by the solid angle,Ω, subtended by the detector, corresponds to 2.4' of arc. Therefore, thecollimator must have an angular resolution of no worse than 2.4' of arc.

Constructing a high angular resolution collimator in accordance with theabove design parameters requires that the material be easy to handle andfabricate as well as have good stopping power to minimize backgroundscatter of the high energy photons.

A preferred method of constructing the high angular resolutioncollimator requires the use of leaded glass micro-channel plate (MCP)detectors. The MCP 36 is an electron multiplier consisting of manybundled channels of optical fibers (microglass capillaries) fused andsliced at their cross section to form a solid core. The solid core takesthe shape of a thin plate or wafer. FIGS. 3 and 4 show a top view and aside view of a MCP, respectively. Each channel has a diameter rangingfrom about 10 to about 20 microns and operates as an independentmultiplier. The preferred diameter of the channels is about 10 microns.The plates typically are about 25 mm in diameter. The plates are thenprocessed chemically by an etching process which selectively etches awayan inner core of glass 38 leaving behind a plate of microcapillaries 40and an outer core 42. The capillaries or channels have a diameter in therange of about 10 to 20 microns. The inner core has a diameter of about18 mm. The microcapillaries 40 are channels comprising very fine holesor pores. Therefore, it can perform electron multiplication whileretaining two-dimensional information. Although MCPs are primarily usedas electron multipliers, their unique properties are ideally suited forcollimator fabrication.

The pore size of the micro-glass capillaries make the fabrication of thehigh resolution collimators feasible. To achieve an angular resolutionof 2.4' of arc as required for this design, an aspect ratio (tube lengthto hole diameter) of about 1500 is needed. This means that for acollimator length of 15 mm, the hole diameter must be 10 microns. MCPsare available with hole diameters of 10 microns, but not with a lengthof 15 mm. The reason being that the etching process for the MCP isdiffusion limited to about 1-2 mm for this hole size.

The present invention has solved this fabrication problem by stacking aplurality of individual plates adjacent to each other. The stack ofindividual plates are aligned in a manner wherein each of the holes fromthe adjacent plates are in exact registration. Without properregistration, the collimator will essentially be closed to the passageof x-rays.

To achieve alignment of the capillary holes from adjacent MCP slices, asolid core of fused glass fibers is provided as shown in FIG. 5a. FIG.5b shows the stack of plates after being partially sliced, leaving aportion of the outer core intact to preserve registration. The bundle iscut only part of the way through (about 0.5 mm), leaving a sufficientthickness of the solid glass bundle intact to provide the necessaryrigidity and alignment. By repeating this process along the length ofthe bundle, the individual cuts define a plurality of leaves 46 allattached to a common base 48 and rigidly held. Since the alignment ofthe fibers in the bundle is nearly perfect over finite lengths, eachleaf in the structure will have excellent registration with the adjacentplates. With narrow cuts between the plates, each plate can bechemically etched independently of the others without disrupting theregistration. FIG. 5c shows the stack of plates as etchant such as ahydrofluoric acid is applied to the inner core to form a plurality ofmicro-capillaries. The etched MCPs are shown in FIG. 5d with thecapillaries shown in precise alignment. Once stacked, the geometry mustbe preserved with some sort of collar or housing as shown in FIG. 6.

Another approach which provides tolerances and mechanical support isslicing all the way through the solid core after forming one or moregrooves along the side of the outer core. All of the cuts made along thelength of the solid core allow each slice to be etched independentlythus eliminating the diffusion problem.

The housing 44 illustrated in FIG. 6 can be provided with one or morealignment ridges along its inner face that mates with the groovesreferenced above. Such a stacked system is shown in FIGS. 7a-c. FIG. 7ashows a flat groove 50. FIG. 7b shows a curved groove 52. FIG. 7c showsa stacked array of plates with grooves 54 in alignment which mate withinternal ridge 56 of the housing 44. Thus, a high resolution collimatorfrom individually stacked MCPs can be constructed. Finally, the glassmaterial is heavily doped with up to 60% lead in the form of lead oxide.After doping, the collimator has the necessary absorptive properties forstopping high energy x-rays impinging outside the solid angle of thecollimator.

The efficiency of this collimator is determined from the product of thesolid angle subtended by a single collimator hole and the fractionaltransparent area of the entrance side of the collimator. The generalrelationship is defined as: ##EQU1## where d is the hole diameter, a isthe collimator length and t is the thickness of the septal wall betweenholes. From the foregoing, it will be seen that a collimator has beenprovided with improved high angular resolution.

Those skilled in the art will know, or be able to ascertain using nomore than routine experimentation, many equivalents to the specificembodiments of the invention described herein. These and all otherequivalents are intended to be encompassed by the following claims.

We claim:
 1. A collimator, comprising:a bundle of optical fibers bondedtogether to form a solid core, the solid core having an inner core andan outer core, the inner core having a plurality of pores therein, theouter core being sliced partially and leaving a portion of the outercore intact to provide a stack of plates with the pores aligned inregistration ensuring passage of an x-ray beam, therethrough; and ahousing enclosing the stack of plates to preserve registration of thepores.
 2. A collimator according to claim 1, wherein the partiallysliced core defines a plurality of individual leaves attached to acommon base.
 3. A collimator according to claim 2, wherein the outercore is sliced about 0.5 mm.
 4. A collimator according to claim 1,wherein the optical fibers are glass.
 5. A collimator according to claim1, wherein the stack of plates have a thickness of about 15 mm.
 6. Acollimator according to claim 1, wherein the stack of plates are dopedwith up to 60% lead oxide.
 7. A collimator according to claim 1, whereinthe inner core has a diameter of about 18 mm.
 8. A collimator accordingto claim 7, wherein the outer core has a diameter of about 25 mm.
 9. Acollimator according to claim 8, wherein the pores have a diameter inthe range of about 10 microns to about 20 microns.
 10. A high angularresolution x-ray collimator, comprising:a plurality of plates stackedadjacent to each other, each plate being formed from a bundle of bondedglass fibers, the stack of plates having an inner core and an outercore, the inner core having a plurality of holes formed by removal of afiber core from each glass fiber such that the holes in each plate arealigned in registration to permit passage of x-rays therethrough; and acollar containing the stack of plates such that registration betweenaligned holes on adjacent plates is maintained.
 11. A high angularresolution x-ray collimator according to claim 10, wherein the stack ofplates has a thickness of about 15 mm.
 12. A high angular resolutionx-ray collimator according to claim 10, wherein the stack of plates aresliced partially along its length.
 13. A collimator according to claim12, wherein the stack of plates are doped with up to 60% lead oxide. 14.A collimator according to claim 13, wherein the partially sliced coredefines a plurality of individual leaves attached to a common base. 15.A collimator according to claim 10, wherein the stack of plates have athickness of about 15 mm.
 16. A collimator according to claim 10,wherein the inner core has a diameter of about 18 mm.
 17. A collimatoraccording to claim 10, wherein the outer core has a diameter of about 25mm.
 18. A collimator according to claim 10, wherein the plurality ofholes within the inner core each have a diameter of about 10 microns toabout 20 microns.
 19. An x-ray diffraction inspection system fordetecting crystalline materials, that are within parcels being inspectedcomprising:a light source irradiating a parcel being inspected with anx-ray beam; a collimator for excluding unwanted x-rays scattered fromthe object, the collimator comprising a plurality of plates stackedadjacent to each other, each plate being formed from a bundle of bondedglass fibers, the stack of plates having an inner core and an outercore, the inner core having a plurality of holes aligned in registrationto ensure passage of wanted x-rays therethrough, and a housingcontaining the stack of plates to provide registration between holes inadjacent plates; and a detector for measuring the intensity of scatteredlight passed through the collimator.
 20. An x-ray diffraction inspectionsystem according to claim 19, wherein the stack of plates has athickness of about 15 mm.
 21. An x-ray diffraction inspection systemaccording to claim 19, wherein the stack of plates are sliced partiallyalong its length.
 22. An x-ray diffraction inspection system accordingto claim 21, wherein the stack of plates are doped with up to 60% leadoxide.
 23. An x-ray diffraction inspection system according to claim 22,wherein the partially sliced core defines a plurality of individualleaves attached to a common base.
 24. An x-ray diffraction inspectionsystem according to claim 19, wherein the stack of plates have athickness of about 15 mm.
 25. An x-ray diffraction inspection systemaccording to claim 19, wherein the inner core has a diameter of about 18mm.
 26. An x-ray diffraction inspection system according to claim 19,wherein the outer core has a diameter of about 25 mm.
 27. An x-raydiffraction inspection system according to claim 19, wherein theplurality of holes within the inner core each have a diameter of about10 microns.
 28. A method of producing a collimator, comprising the stepsof:fusing a bundle of optical fibers; cutting the bundle of opticalfibers along its length to make a plurality of plates; etching an innercore within the plates to remove a portion of each optical fiber toprovide holes extending through each plate; and stacking the pluralityof plates to align the holes through adjacent plates such that the holesare in registration.
 29. A method according to claim 28, furthercomprising the step of doping the plates in lead oxide.
 30. A methodaccording to claim 28, wherein the bundle of fibers is partially cuttherethrough.
 31. A method according to claim 30, wherein the bundle iscut about 0.5 mm.
 32. A method according to claim 28, wherein the stackof plates have a thickness of about 15 mm.
 33. A method according toclaim 28, wherein the inner core has a diameter of about 18 mm.
 34. Amethod according to claim 28, wherein the plurality of holes within theinner core each have a diameter of about 10 microns.
 35. A methodaccording to claim 28, further comprising the step of machining a flatin the bundle of fibers before cutting.
 36. A method according to claim28, further comprising the step of preserving the etched plates in exactregistration.
 37. A method according to claim 28 wherein optical fibersare made of glass.
 38. A method of producing a collimator, comprisingthe steps of:fusing a bundle of optical fibers; machining a flat in thebundle of optical fibers; cutting the bundle of optical fibers along itslength forming individual plates; etching an inner core of each plate toremove material from the optical fibers to form holes extending througheach plate; stacking the plates adjacent to each other; aligning theplates such that the holes through adjacent plates are in registration;housing the stacked plate to preserve registration; and doping thehousing with lead oxide.
 39. A method according to claim 38, wherein theoptical fibers are made of glass.
 40. A method according to claim 38,wherein the bundle of fibers is partially cut therethrough.
 41. A methodaccording to claim 38, wherein the bundle is cut about 0.5 mm.
 42. Amethod according to claim 38, wherein the stack of plates have athickness of about 15 mm.
 43. A method according to claim 38, whereinthe inner core has a diameter of about 18 mm.
 44. A method according toclaim 38, wherein the plurality of holes within the inner core each havea diameter of about 10 microns.
 45. A method of x-ray diffractioninspection for detecting crystalline materials within parcels beinginspected, comprising:irradiating a parcel with an x-ray beam;collimating x-rays from the parcel with a collimator which excludesunwanted x-rays scattered from the object, the collimator comprising aplurality of plates stacked adjacent to each other, each plate beingformed from a bundle of bonded glass fibers, the stack of plates havingan inner core and an outer core, the inner core having a plurality ofholes aligned in registration to permit passage of x-rays through thealigned holes, and a housing containing the stack of plates to preserveregistration; and detecting the intensity of scattered light passedthrough the collimator.
 46. The method of claim 45, further comprisingproviding the stack of plates having a thickness of about 15 mm.
 47. Themethod of claim 45, further comprising providing the stack of plateswhich are sliced partially to form gaps between the plates.
 48. Themethod of claim 45 wherein the crystalline material comprises anarcotic.
 49. The method of claim 45 wherein the crystalline materialcomprises an explosive.